This invention relates to gate drive circuits for thyristor deflection systems such as used in television receivers.
A two bidirectional conducting switch deflection system is described in U.S. Pat. No. 3,452,244. Such a deflection system includes two switches each comprised of a diode coupled in parallel with a thyristor (SCR) and poled for forward conduction in opposite directions. A first switch is a commutating switch operative each deflection cycle to store energy in an inductor which energy is then transferred to other reactive components in the deflection circuit to replenish losses incurred by the supplying of scanning current to a deflection winding from a second switch during the trace portion of each deflection cycle.
The commutating and trace thyristors must be enabled for conduction during different portions of each deflection cycle. It has been common practice to gate the commutating thyristor with a pulse obtained from a horizontal rate oscillator. The trace switch thyristor commonly is gated by waveshaping a voltage obtained from the commutating circuit. Although an oscillator may be designed to provide the differently timed commutating and trace switch drive waveforms during each deflection cycle, this arrangement may not be the most efficient from the point of view that the oscillator would have to be specially designed for this purpose and would then not be compatible for use in alternative horizontal deflection systems employing a single output transistor.
Arrangements have been devised for utilizing the horizontal deflection circuit to provide chassis isolation from the input power line. This arrangement is desirable from the point of view of safety and for providing isolation from the line so that portions of the television receiver such as the video portions may be attached to remote video sources without the possibility of electrical shock to the user.
Since a thyristor deflection system of the type described in the aforementioned patent is suitable for supplying alternating current which may be rectified to provide direct current operating potential for the rest of the receiver, isolation of the deflection output stage is extremely desirable not only from the point of view of safety but also because of the cost saving achieved by eliminating the use of a separate isolation transformer on the incoming power line.
One point in the thyristor deflection system at which isolation may be obtained is the coupling between the commutating and trace switches by means of a transformer which may be a part of the horizontal output transformer for cost saving reasons. In such a situation a problem arises in driving the trace switch from the commutating circuit because the trace switch is isolated whereas the commutating switch is not. It is desirable to keep the number of connections such as transformer connections between the isolated and non-isolated portions of the deflection system to a minimum. Hence, an alternative way to the transformer coupling arrangement or the second oscillator output arrangement for driving the trace thyristor is desirable.